Chronic hormonal control of lipid synthesis and hydrolysis in adipocytes
The aim of this study was to. further elucidate the mechanisms whereby growth
. hormone (GH) exerts its ~hronic effect on adipose tissue metabolism, and in particular
the effects of GH on lipolysis. Previous studies had shown that turnout necrosis factor
alpha (TNFa.) chronically increases basal lipolysis in rat epididymal cidipocytes: an
effect that is similar to that of GH. Other research had implicated a protein with a halflife
of less than 3 hours in the regulation of lipolysis and lipogenesis by GR. This led
to the investigation if TNFa. might be this putative protein involved in mediating the
chronic metabolic effects of GH. Initial studies used ovine adipose tissue explants.
TNFa. caused a small increase in basal lipolysis and attenuated insulin effects on
lipogenesis. However, the effects ofTNFa. were smaller than those ofGH and TNFa.
did not appear to mimic the effects of GH on isoproterenol-stimulated lipolysis in this
system. Therefore TNFa. was not the protein involved in GH regulation of lipolysis
Previous studies in the laboratory on the mechanism of GH action had used inhibition
of signal transduction components in an ex.plant system. More specific effects could be
observed by using an antisense approach, but this required the use of a cell culture
system rather than adipose tissue explants. The suitability of an ovine cell culture
system was established for investigating the molecular basis of the lipolytic effects of
GH; in particular the inhibitory effects of GH on adenosine inhibition of lipolysis. The
lipolytic system partially developed in primary ovine adipocytes, but the antilipolytic
system did not appear. to develop. However, by. manipulating the differentiation
conditions, I significantly improved both cell differentiation and the lipolytic response
and sensitivity to isoproterenol, but there· was no improvement in response to
adenosine. As an alternative, the suitability of the murine cell line, 3T3-F442A, was
investigat.ed ·for determining the molecular basis of the lipolytic effects of GH.
However, although the lipolytic system did· develop in differentiated 3T3-F442A
adipocytes and response to isoproterenol was observed, the antilipolytic system did not
appear io develop either. This line of investigation was not pursued further.
Therefore, I decided to investigate the effects of GH and insulin on the lipogenic
system in 3T3-F442A adipocytes i.nstead, with a view to extending previous
observations by others (Millar, 1998) in the laboratory on the roles of specific isoforms
of protein kinase C (PKC) on the modulation of lipogenesis by insulin and GH. The
main objective was to determine the role of PKC isoforms in the modulation of the
effect of insulin and GH on activation' and expression (mRNA) of the lipogenic enzyme
. acetyl CoA carboxylase (ACC). However, the effect of the hormones on lipogenesis,
and especially ACC, was considered to be too small to investigate the roles of specific
PKC isoforms, despite trying many different ways of improving the hormone effects.
A possible explanation for the poor response to insulin was that the lipogenic system
was not "switching off' in the absence of insulin, so isoproterenol was added to the
3T3-F442A adipocytes to decrease lipogenesis. Isoproterenol did reduce the rate of
lipogenesis, but the effect of insulin was still small. Therefore, modulation of the effect
of specific phosphodiesterase (POE) isoforms on lipogenesis was explored as an
alternative. The use of specific POE inhibitors showed that both POE3 and PDE4
enzymes were involved in the modulation of lipogenesis in 3T3-F442A adipocytes.